NO ONE who grasps the seriousness of the missile and nuclear weapons threat from North Korea can dismiss the significance of a unanimous vote of the 15-member U.N. Security Council, including China, Russia and the United States, for yet another round of sanctions, the eighth in 11 years. The vote reflects a broad sense of international alarm, which reached new levels after the July 4 and July 28 tests of ballistic missiles that could reach the United States. The vote is very welcome.

But going beyond alarm to effective action has long been the hard part and is not getting any easier. Every military option carries the risk of setting off a devastating war; regime change, which would be the best outcome for the long-suffering North Korean people, does not appear to be imminent. What remains is some combination of persuasion, negotiation and coercion. The target of this is a belligerent leader of a country that has, over decades, repeatedly negotiated in bad faith. Not simple at all.

The latest round of sanctions prohibits North Korea from buying, selling or transferring coal, iron, iron ore, seafood, lead and lead ore to other countries, and attempt to restrict North Korean labor abroad, among other things. By some estimates, if fully implemented, the punishment would cut North Korea’s foreign earnings by $1 billion, or about a third.

However, sanctions are a blunt instrument and can take a long time to have any effect. The sanctions on North Korea, first imposed after the 2006 nuclear test and significantly broadened in 2016, have so far had little dis­cern­ible impact. Why? Implementation has been spotty and sometimes miserable. Andrea Berger, in a recent report for the Royal United Services Institute, a British think tank, says the U.N. sanctions on North Korea are a “house without foundations.” She adds, “The narrative around the UN Security Council table that sanctions are the ‘strongest’ they have ever been may be true of their paper form, but is fiction in practice.” The problem, she notes, is that North Korea exploits illicit supply networks, individual states don’t implement sanctions fully and private-sector firms can often undermine them. “Gaps allow North Korean illicit activity to persist,” she says. So far, the Security Council has not taken the full plunge to choke off all economic activity that allows Kim Jong Un’s regime to carry on.

President Trump seems to grasp the dangers of North Korea’s expanding nuclear and missile programs, but it is not clear what he intends to do, aside from his tweeted broadsides at China. Beijing’s role in any solution is large but not singular. This is the kind of security problem that requires deft diplomacy and alliance-building — not the forte of this administration, at least so far. New sanctions are a necessary and potentially useful precondition, but what are the next steps to bring the bellicose North Korean leader to negotiate a verifiable agreement to stop his nuclear and missile programs? We have yet to see a coherent strategy. Nor has Mr. Kim felt the heat.